1. Field of the Invention
The present invention relates to a differential expansion absorption mechanism for absorbing differential thermal expansion between members, and a fuel injection valve comprising same.
2. Description of the Related Art
Problems shared by mechanisms having comparatively elongated members (for example, an elongated actuator, rod, or the like) include physical deviations, malfunctions, and so on caused by differential thermal expansion between members. The reason for this is that when a member is elongated, differential thermal expansion (a difference in dimensional change caused by thermal expansion or thermal contraction) due to a temperature difference or a difference in the coefficient of thermal expansion (difference in material) between members increases.
Examples of a mechanism comprising an elongated member include a fuel injection valve mounted on a cylinder head or the like of an engine.
As shown in FIG. 7, for example, a fuel injection valve 100 for injecting a gaseous fuel, which is currently under development by the present inventor and so on, comprises a cylinder 102 accommodated movably (slidably) within a comparatively elongated barrel 101, a piston 105 accommodated movably (slidably) within the cylinder 102 so as to partition the interior of the cylinder 102 into an upper chamber 103 and a lower chamber 104, an incompressible viscous fluid (illustrated by dots) charged into the upper chamber 103 and lower chamber 104 respectively, an actuator 106 for raising the cylinder 102, and a needle valve 107 joined integrally to the piston 105. When the cylinder 102 is raised by the actuator 106, the needle valve 107 is lifted via the viscous fluid in the lower chamber 104 and the piston 105, thereby opening an injection hole 108 formed on the leading end (lower end) of the barrel 101.
The barrel 101 comprises a barrel main body 109, a tip 110 mounted on the lower end of the barrel main body 109 via a lock nut 119, and a cap 112 screwed onto the upper end of the barrel main body 109. The aforementioned fuel injection hole 108 is formed in the lower end of the tip 110, and a fuel inlet 111 is formed in the cap 112.
The cylinder 102 is supported and accommodated within the barrel main body 109 so as to be capable of sliding in a longitudinal direction (up/down direction). The cylinder 102 is constituted by a cylinder main body 117 in closed-end cylinder form, and a cylinder cap 118 which is screwed onto, and thus covers, the upper portion of the cylinder main body 117.
The piston 105 is accommodated within the cylinder 102 so as to be capable of sliding in the same direction (up/down direction) as the sliding direction of the cylinder 102 within the barrel 101, and the incompressible viscous fluid is charged into the upper chamber 103 and lower chamber 104 partitioned by the piston 105. The viscous fluid is charged through an injection passage not shown in the drawing such that the interior of the upper chamber 103 and lower chamber 104 is completely deaerated. The viscous fluid injection passage is blocked by a plug or the like after the viscous fluid has been injected.
The needle valve 107 is joined to the lower surface of the piston 105. The needle valve 107 extends downward through a through hole 128 provided in a bottom wall of the cylinder main body 117 such that the lower end thereof abuts against a seat portion 125 formed in the interior of the leading end of the barrel 101. The through hole 128 is provided with a sealing member 129 (an O-ring, for example) for sealing the gap between the through hole 128 and needle valve 107 in a fluid-tight fashion. Further, the fuel injection valve 100 is designed such that fuel supplied to the barrel 101 from the fuel inlet 111 provided in the upper end of the barrel 101 flows past each member into the seat portion 125.
A rod 120 is provided on the upper surface of the piston 105. The rod 120 is inserted slidably into a through hole 130 formed in the cylinder cap 118, and urged downward by a plate spring 123 via a pressing member (intermediate member) 122. The through hole 130 is provided with a sealing member 131 (an O-ring, for example) for sealing the gap between the through hole 130 and rod 120 in a fluid-tight fashion. By urging the needle valve 107 downward using the plate spring 123, the lower end portion of the needle valve 107 is seated on the seat portion 125 at a predetermined pressure, thereby closing the injection hole 108.
The actuator 106 is provided between the needle valve 107 and barrel main body 109. The actuator 106 comprises a magnetostrictor 113 disposed on the outside of the needle valve 107, and a coil 114 disposed on the outside of the magnetostrictor 113. The lower end of the magnetostrictor 113 abuts against a stepped surface portion 132 within the barrel main body 109 via a seat 115, and the upper end abuts against a lower surface of the cylinder main body 117 via a seat 116.
A plate spring 121 which urges the cylinder 102 downward to press the cylinder 102 against the magnetostrictor 113 via the seat 116 is disposed above the cylinder 102. The urging force of this plate spring 121 is greater than the urging force of the plate spring 123.
When the coil 114 of the actuator 106 is not energized via an external terminal 126 provided on the barrel 101, the needle valve 107 is urged downward by the plate spring 123, and hence the lower end portion of the needle valve 107 is pressed against the seat portion 125 of the tip 110 at a predetermined pressure such that the injection hole 108 is closed. Accordingly, fuel does not reach the injection hole 108, and fuel injection is not performed.
On the other hand, when the coil 114 is energized via the external-terminal 126, the coil 114 is magnetized, and the magnetostrictor 113 elongates in accordance with the magnetic force (magnetic field). At this time, the lower end of the magnetostrictor 113 is in contact with the stepped surface portion 132 of the barrel main body 109 via the seat 115, and hence the magnetostrictor 113 elongates in such a manner as to push the cylinder 102 upward against the urging force of the plate spring 121. When the cylinder 102 is pushed upward, the piston 105 and needle valve 107 are raised (lifted) integrally via the viscous fluid in the lower chamber 104. As a result, the lower end of the needle valve 107 separates from the seat portion 125 of the tip 110, thereby opening the fuel injection hole 108, and thus fuel injection is performed.
This type of fuel injection valve is also disclosed in Japanese Translation of International Patent Application Publication 2003-512555, for example.
With this type of fuel injection valve 100, the length (the dimension in the up/down direction) of the magnetostrictor 113 must be increased to a certain extent to secure the maximum lift amount required of the needle valve 107. As a result, the dimensions of the barrel 101, needle valve 107, and so on must be lengthened in alignment with the dimension of the magnetostrictor 113.
As described above, with a mechanism comprising an elongated member, differential thermal expansion between components (a difference in dimensional change due to thermal expansion or thermal contraction) is problematic. Particularly with the fuel injection valve 100, the lift amount of the needle valve 107, or in other words the amount of displacement of the actuator 106 (the elongation amount of the magnetostrictor 113) is comparatively small (several tens of μm, for example), and therefore even slight differential thermal expansion may affect operations.
Hence in the fuel injection valve 100 shown in FIG. 7, when differential thermal expansion occurs between members, measures are taken to enable the viscous fluid to move between the upper chamber 103 and lower chamber 104 through a small gap (clearance) between the inner surface of the cylinder 102 and the outer surface of the piston 105.
For example, when the thermal expansion of the magnetostrictor 113 is greater than the thermal expansion of the needle valve 107, a force which raises the cylinder 102 at a much lower speed than the driving speed of the actuator 106 (the elongation speed of the magnetostrictor 113 generated by change in the magnetic field) is produced, but at this time, the viscous fluid in the lower chamber 104 moves into the upper chamber 103 through the clearance between the cylinder 102 and piston 105. This causes the cylinder 102 to move upward relative to the piston 105 such that the differential thermal expansion between the needle valve 107 and magnetostrictor 113 is absorbed. As a result, the positions of the piston 105 and needle valve 107 become constant, and the operation is not affected.
Conversely, when the cylinder 102 is lifted upward by elongating the magnetostrictor 113 in order to perform fuel injection through the injection hole 108, the cylinder 102 is raised at a much higher speed than the aforementioned speed, and hence the pressure increase speed of the viscous fluid in the lower chamber 104 rises greatly beyond the pressure increase speed during the thermal expansion described above. At this time, the viscous fluid in the lower chamber 104 functions as a solid, and does not move to the upper chamber 103 through the clearance between the cylinder 102 and piston 105. Instead, the piston 105 and needle valve 107 are lifted integrally with the cylinder 102, and thus fuel injection is performed.
However, with the fuel injection valve 100, in which the viscous fluid is moved through the clearance between the cylinder 102 and piston 105 in the manner described above, a problem exists in that differences arise in the differential thermal expansion absorption performance of individual products (individual fuel injection valves).
The following points may be cited as reasons for this.
Reason 1: Differences in the clearance between the inner surface of the cylinder 102 and the outer surface of the piston 105 occur among individual products due to the difficulty involved in controlling and managing the clearance to a high degree of precision. Measures which may be taken to avoid this problem include increasing the finishing precision of the cylinder 102 and piston 105 or equalizing the clearance by measuring the dimensions of the cylinder 102 and piston 105 and selecting appropriate combinations thereof, but when such measures are implemented, adverse effects on productivity, such as cost increases and labor increases, are inevitable.
Reason 2: Variation in the cylindricity (circularity) of the inner surface of the cylinder 102 and the outer surface of the piston 105, variation (offset) in the concentricity of the cylinder 102 and piston 105, variation (tilting) between the central axis of the cylinder 102 and the central axis of the piston 105, and so on differ among individual products, and as a result, differences occur in the clearance of each product.
Reason 3: Dimensional change over time due to the sliding and so on of the cylinder 102 and piston 105 differs among individual products, and hence with use, differences in the clearance of individual products increase.
Reason 4: The viscosity of the viscous fluid changes due to wear particles produced by the sliding of the cylinder 102 and piston 105 entering the viscous fluid, and this change in viscosity differs among individual products. As a result, variation in the differential thermal expansion absorption performance occurs with use.
The fuel injection valve 100 described above also has the following problems.
In the fuel injection valve 100, the total volume of the upper chamber 103 and lower chamber 104 in the cylinder 102 is constant even when the piston 105 moves. Hence when the viscous fluid thermally expands to a greater extent than the cylinder 102, the pressure of the viscous fluid in the cylinder 102 increases, leading to such problems as disengagement or cracking of the sealing members 129, 131 such that the viscous fluid flows out of the upper chamber 103 and lower chamber 104, or disengagement of the plug which blocks the injection passage for injecting the viscous fluid such that the viscous fluid flows out therefrom.
To describe this point in further detail, in actuality, the change in the volume of the viscous fluid caused by thermal expansion thereof differs from the change in the total volume of the upper chamber 103 and lower chamber 104 caused by thermal expansion of the cylinder 102 by close to two orders of magnitude. Hence, for example, when the viscous fluid and cylinder 102 rise to a substantially equal temperature due to an increase in the overall temperature of the fuel injection valve 100 caused by heat from the cylinder head or the like, the thermal expansion of the viscous fluid is great, whereas the cylinder 102 does not thermally expand to a large extent. As a result the total volume of the upper chamber 103 and lower chamber 104 does not increase greatly, and therefore the basically incompressible viscous fluid tries to find an escape route out of the upper chamber 103 and lower chamber 104.
Here, the upper chamber 103 and lower chamber 104 are completely deaerated, and hence the internal pressure of the cylinder 102 increases, causing the expanded viscous fluid to break through, and flow out from, the comparatively weak sealing members 129, 131 for forming the upper chamber 103 and lower chamber 104 into airtight spaces, and/or the plug blocking the injection passage, and/or other possible fluid outlets. Note that the reason for completely deaerating the upper chamber 103 and lower chamber 104 is that if air bubbles existed within the upper chamber 103 and lower chamber 104, the air bubbles would be compressed upon elongation of the magnetostrictor 113 elongates in order to raise the cylinder 102. As a result the piston 105 would not rise integrally with the cylinder 102, leading to a delay or difficulty in lifting the needle valve 107.
To prevent such overflowing of the viscous fluid due to thermal expansion thereof, components having a substantially equal coefficient of thermal expansion may be used for the viscous fluid and cylinder 102. In reality, however, almost no such components exist. With the actual materials and substances used as the viscous fluid and cylinder 102, a differential thermal expansion of at least one order of magnitude exists between the viscous fluid and cylinder 102.